Heat pipe and method of making the same

ABSTRACT

A heat pipe includes a casing, a main wick structure received in the casing and attached to an inner surface of the casing, a multi-layered auxiliary wick structure received in the main wick structure and a working fluid contained in the casing and saturating the main wick structure and the auxiliary wick structure. An inner peripheral surface of the main wick structure and an outer peripheral surface of the auxiliary wick structure cooperatively define a vapor channel therebetween. The auxiliary wick structure extends along a longitudinal direction of the casing and defines a liquid channel therein. The auxiliary wick structure is formed by a plurality of layers radially stacked on each other, such that each outer layer is attached around an adjacent inner layer.

BACKGROUND

1. Technical Field

The present invention relates generally to an apparatus for transfer ordissipation of heat from heat-generating components, and moreparticularly to a heat pipe applicable in electronic products such aspersonal computers for removing heat from electronic componentsinstalled therein and a method for manufacturing the same.

2. Description of Related Art

Heat pipes have excellent heat transfer performance due to their lowthermal resistance, and are therefore an effective means for transfer ordissipation of heat from heat sources. Currently, heat pipes are widelyused for removing heat from heat-generating components such as centralprocessing units (CPUs) of computers. A heat pipe is usually a vacuumcasing containing therein a working medium, which is employed to carry,under phase transitions between liquid state and vapor state, thermalenergy from one section of the heat pipe (typically referring to as the“evaporator section”) to another section thereof (typically referring toas the “condenser section”). Preferably, a wick structure is providedinside the heat pipe, lining an inner wall of the casing, for drawingthe working medium back to the evaporator section after it is condensedat the condenser section. The wick structure currently available for theheat pipe includes fine grooves integrally formed at the inner wall ofthe casing, screen mesh or fiber inserted into the casing and heldagainst the inner wall thereof, or sintered powders combined to theinner wall of the casing by sintering process.

In operation, the evaporator section of the heat pipe is maintained inthermal contact with a heat-generating component. The working mediumcontained at the evaporator section absorbs heat generated by theheat-generating component and then turns into vapor. Due to thedifference of vapor pressure between the two sections of the heat pipe,the generated vapor moves and thus carries the heat towards thecondenser section where the vapor is condensed into condensate afterreleasing the heat into ambient environment by, for example, finsthermally contacting the condenser section. Due to the difference incapillary pressure which develops in the wick structure between the twosections, the condensate is then brought back by the wick structure tothe evaporator section where it is again available for evaporation.

In order to draw the condensate back timely, the wick structure providedin the heat pipe is expected to provide a high capillary force andmeanwhile generate a low flow resistance for the condensate. In ordinaryuse, the heat pipe needs to be flattened to enable the miniaturizationof electronic products, which results in the wick structure of the heatpipe being damaged. Therefore, the flow resistance of the wick structureis increased and the capillary force provided by the wick structure isdecreased, which reduces the heat transfer capability of the heat pipe.If the condensate is not quickly brought back from the condensersection, the heat pipe will suffer a dry-out problem at the evaporatorsection.

Therefore, it is desirable to provide a heat pipe with an improved heattransfer capability, whose wick structure will not be damaged when theheat pipe is flattened.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present embodiments can be better understood withreference to the following drawings. The components in the drawings arenot necessarily drawn to scale, the emphasis instead being placed uponclearly illustrating the principles of the present embodiments.Moreover, in the drawings, like reference numerals designatecorresponding parts throughout the several views.

FIG. 1 is a longitudinal cross-sectional view of a heat pipe inaccordance with a first embodiment of the present invention.

FIG. 2 is a transverse cross-sectional view of the heat pipe of FIG. 1.

FIG. 3 is a flow chart showing a method for manufacturing the heat pipeof FIG. 1.

FIG. 4 is a transverse cross-sectional view of a heat pipe in accordancewith a second embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a heat pipe 10 includes an elongated, roundcasing 12 containing a working fluid therein, a main wick structure 14and an auxiliary wick structure 18.

The casing 12 is made of a highly thermally conductive material such ascopper or aluminum. The casing 12 includes an evaporator section 121, anopposing condenser section 122, and an adiabatic section 123 disposedbetween the evaporator section 121 and the condenser section 122.

The main wick structure 14 is tube-shaped in profile, which is evenlydistributed around and attached to an inner surface of the casing 12.The main wick structure 14 defines a receiving space therein. The mainwick structure 14 extends along a longitudinal direction of the casing12. The main wick structure 14 is usually selected from a porousstructure such as fine grooves, sintered powder, screen mesh, or bundlesof fiber, and provides a capillary force to drive condensed workingfluid at the condenser section 122 to flow towards the evaporatorsection 121 of the heat pipe 10.

The auxiliary wick structure 18 is a longitudinal hollow tube, which isreceived in the receiving space of the main wick structure 14 andextends along the longitudinal direction of the casing 12. The auxiliarywick structure 18 has a ring-like transverse cross section. Theauxiliary wick structure 18 longitudinally defines a liquid channel 172therein. An outer diameter of the auxiliary wick structure 18 is muchsmaller than a bore diameter of the main wick structure 14.

The auxiliary wick structure 18 is a multi-layered structure, which isoutwardly and radially formed by a plurality of round layers such thateach successive layer is attached to a previous layer. In theembodiment, the auxiliary wick structure 18 includes a first layer 181at an inner side and a second layer 182 at an outer side and attachedimmediately around the first layer 181. An inner peripheral surface ofthe first layer 181 longitudinally defines the liquid channel 172therein. An inner peripheral surface of the main wick structure 14 andan outer peripheral surface of the second layer 182 of the auxiliarywick structure 18 cooperatively define a vapor channel 171 in the casing12. The outer peripheral surface of the auxiliary wick structure 18 hasa bottom side 164 contacting with the inner peripheral surface of themain wick structure 14, and a top side 165 spaced from the innerperipheral surface of the main wick structure 14.

The first and the second layers 181, 182 are formed by weaving aplurality of metal wires, such as copper wires. A plurality of pores isformed in the first and the second layer 181, 182, which provides acapillary action to the working fluid. The metal wires of the firstlayer 181 has a greater wire diameter than that of the metal wires ofthe second layer 182, whereby the metal wires of the first layer 181have a greater mechanical strength to support the whole auxiliary wickstructure 18, which prevents the auxiliary wick structure 18 fromcollapsing down to thereby maintain the intended shape of the pores andthe liquid channel 172. Moreover, since the metal wires of the secondlayer 182 have a smaller wire diameter than the metal wires of the firstlayer 181, the second layer 182 has a smaller pore size than the firstlayer 181, whereby the second layer 182 has a greater capillary actionto absorb more working fluid.

The working fluid is saturated in the main and the auxiliary wickstructures 14, 18 and is usually selected from a liquid such as water,methanol, or alcohol, which has a low boiling point and is compatiblewith the main and the auxiliary wick structures 14, 18. Thus, theworking fluid can easily evaporate to vapor when it receives heat at theevaporator section 121 of the heat pipe 10.

In operation, the evaporator section 121 of the heat pipe 10 is placedin thermal contact with a heat source, for example, a central processingunit (CPU) of a computer, which needs to be cooled. The working fluidcontained in the evaporator section 121 of the heat pipe 10 is vaporizedinto vapor upon receiving the heat generated by the heat source. Then,the generated vapor moves via the vapor channel 171 towards thecondenser section 122 of the heat pipe 10. After the vapor releases theheat carried thereby and is condensed into the condensate in thecondenser section 122, the condensate is brought back by the main wickstructure 14 and the auxiliary wick structure 18 to the evaporatorsection 121 of the heat pipe 10 for being available again forevaporation.

Referring to FIG. 3, a method for manufacturing the heat pipe 10includes the following steps: providing an elongated pole, and weaving aplurality of first metal wires on an outer peripheral surface of thepole to form the first layer 181; weaving a plurality of second metalwires on an outer peripheral surface of the first layer 181 to form asecond layer 182; removing the pole from the first layer 181 to form theauxiliary wick structure 18, wherein the first layer 181 defines theliquid channel 172 therein; providing a casing 12 having a main wickstructure 14 attached to an inner peripheral surface thereof, insertingthe auxiliary wick structure 18 into the casing 12; vacuuming aninterior of the casing 12 and filling the working fluid into the casing12; and sealing the casing 12.

Table 1 below shows an average of maximum heat transfer rates (Qmax) andan average of heat resistances (Rth) of forty-five conventional roundgrooved heat pipes and forty-five round heat pipes 10 formed inaccordance with the present disclosure. Qmax represents the maximum heattransfer rate of the heat pipe at an operational temperature of 50° C.Rth is obtained by dividing the margin between an average temperature ofthe evaporator section 121 and an average temperature of the condensersection 122 of the heat pipe 10 by Qmax. A diameter of the transversecross section and a longitudinal length of each of the conventionalgrooved heat pipes are 6 mm and 160 mm, which are respectively equal tothe transverse diameter and the longitudinal length of each of thepresent heat pipes 10. Table 1 shows that the heat resistance of thepresent round heat pipe 10 is significantly less than that of theconventional round grooved heat pipe, whilst the Qmax of the round heatpipe 10 in accordance with the present disclosure is significantly morethan that of the conventional round grooved heat pipe.

TABLE 1 average of Qmax average of Rth Types of heat pipes (unit: w)(unit: ° C./w) Conventional grooved 65 0.025 heat pipes present heatpipes 95.5 0.024

As shown in FIG. 4, a flat heat pipe 50 in accordance with a secondembodiment of the present invention is obtained by flattening the heatpipe 10 of FIGS. 1 and 2. The heat pipe 50 has the same structure as theheat pipe 10 except that the heat pipe 50 is a flat one. After theflattening operation, the auxiliary wick structure 18 is kept intact,and the auxiliary wick structure 18 spaces a gap from a top wall 52 ofthe heat pipe 50. The heat transfer capability of the flat heat pipe 50is not decreased due to the flattening operation. The heat transfercapability of the flat heat pipe 50 is better than a conventional flatheat pipe whose wick structure is damaged in the flattening operation.

Table 2 below shows an average of maximum heat transfer rates (Qmax) andan average of heat resistances (Rth) of ten conventional flat groovedheat pipes and ten present heat pipes 50, which are flattened to have aheight of 3.5 mm. Before these heat pipes are flattened, they have thesame transverse diameter and longitudinal length as the heat pipesmentioned in Table 1. Qmax and Rth in Table 2 have the same meaning asthe Qmax and Rth in Table 1. Table 2 shows that the heat resistance ofthe present flat heat pipe 50 is significantly less than that of theconventional flat grooved heat pipes, whilst the Qmax of the flatpresent heat pipes 50 is significantly more than that of theconventional flat grooved heat pipes.

TABLE 2 average of Qmax average of Rth Types of heat pipes (unit: w)(unit: ° C./w) Conventional grooved 32 0.055 heat pipes Present heatpipes 64 0.033

It is believed that the present embodiments and their advantages will beunderstood from the foregoing description, and it will be apparent thatvarious changes may be made thereto without departing from the spiritand scope of the invention or sacrificing all of its materialadvantages, the examples hereinbefore described merely being preferredor exemplary embodiments of the invention.

1. A heat pipe comprising: a casing; a main wick structure received inthe casing and attached to an inner surface of the casing; amulti-layered auxiliary wick structure received in the main wickstructure, an inner peripheral surface of the main wick structure and anouter peripheral surface of the auxiliary wick structure cooperativelydefining a vapor channel therebetween, the auxiliary wick structureextending along a longitudinal direction of the casing and defining aliquid channel therein; and a working fluid contained in the casing andsaturated in the main wick structure and the auxiliary wick structure;wherein the auxiliary wick structure is formed by a plurality of layersradially disposed on each other in which each outer layer is attachedimmediately around an adjacent inner layer.
 2. The heat pipe as claimedin claim 1, wherein the auxiliary wick structure comprises a first innerlayer and a second outer layer attached around the first layer, an innersurface of the first inner layer defining the liquid channel, the innersurface of the main wick structure and an outer peripheral surface ofthe auxiliary wick structure defining the vapor channel.
 3. The heatpipe as claimed in claim 1, wherein each layer of the auxiliary wickstructure is formed by weaving a plurality of wires.
 4. The heat pipe asclaimed in claim 3, wherein the wires of an inner layer have a greaterwire diameter than that of the wires of an adjacent outer layer of theauxiliary wick structure.
 5. The heat pipe as claimed in claim 1,wherein an outer diameter of the auxiliary wick structure is smallerthan a bore diameter of the main wick structure.
 6. The heat pipe asclaimed in claim 1, wherein the casing is round.
 7. The heat pipe asclaimed in claim 1, wherein the casing is flattened.
 8. A method formanufacturing a heat pipe comprising the steps of: providing anelongated pole, weaving a plurality of first wires on an outerperipheral surface of the pole to form a first layer; weaving aplurality of second wires on an outer peripheral surface of the firstlayer to form a second layer; removing the pole from the first layer toform an auxiliary wick structure, the auxiliary wick structure defininga liquid channel therein; providing a casing having a main wickstructure, the main wick structure attached to an inner surface of thecasing, inserting the auxiliary wick structure into the casing;vacuuming the casing and filling a working fluid into the casing; andsealing the casing.
 9. The method as claimed in claim 8, wherein theplurality of first wires of the first layer has a greater wire diameterthan that of the plurality of second wires of the second layer.